1. Field of the Invention
The present invention generally relates to a spine formation device to form a spine of a bundle of folded sheets, a post-processing apparatus including the spine formation device, and a spine formation system including the spine formation device, and an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine capable of at least two of these functions.
2. Discussion of the Background Art
Post-processing apparatuses to perform post processing of recording media, such as aligning, sorting, stapling, punching, and folding of sheets, are widely used and are often disposed downstream from an image forming apparatus to perform post-processing of the sheets output from the image forming apparatus. At present, post-processing apparatuses generally perform saddle-stitching along a centerline of sheets in addition to conventional edge-stitching along an edge portion of sheets.
To improve the quality of the finished product, several approaches, described below, for shaping the folded portion of a bundle of saddle-stitched sheets have been proposed. More specifically, when a bundle of sheets (hereinafter “booklet”) is saddle-stitched and then folded in two, its folded portion, that is, a portion around its spine, tends to bulge, degrading the overall appearance of the booklet. In addition, because the bulging spine makes the booklet thicker on the spine side and thinner on the opposite side, when the booklets are piled together with the bulging spines on the same side, the piled booklets tilt more as the number of the booklets increases. Consequently, the booklets might fall over when piled together.
By contrast, when the spine of the booklet is flattened, bulging of the booklet can be reduced, and accordingly multiple booklets can be piled together. This flattening is important for ease of storage and transport because it is difficult to stack booklets together if their spines bulge, making it difficult to store or carry them. With this reformation, a relatively large number of booklets can be piled together.
The bulging spine of the booklet can, for example, be flattened using a pressing member configured to sandwich the portion adjacent to the spine of the booklet and a spine-forming roller configured to roll along that side of the pressing member from which the spine of the booklet protrudes in a longitudinal direction of the spine of the booklet while contacting the spine of the booklet. The spine-forming roller moves at least once over the entire length of the spine of the booklet being fixed by the pressing member while applying to the spine a pressure sufficient to flatten the spine.
However, because only the bulging portion is pressed with the spine-forming roller in this approach, the booklet can wrinkle in a direction perpendicular to the longitudinal direction in which the spine extends, degrading its appearance. In addition, with large sheet sizes, productivity decreases because it takes longer for the spine-forming roller to move over the entire length of the spine of the booklet.
Alternatively, a center portion of the saddle-stitched booklet in a direction in which the booklet is transported (hereinafter “sheet conveyance direction”) may be pushed with a folding plate so that the booklet is sandwiched between a first pair of rollers, thereby forming the spine. With the booklet kept at a predetermined position, a second pair of rollers that move in a direction perpendicular to the sheet conveyance direction presses the folded portion from the side. In this approach, differently from the above-described approach, not the spine in parallel to a thickness direction of the booklet but the portion perpendicular to the spine is pressed, thus increasing the pressure per unit length. As a result, the spine can be shaped better, improving the quality of the booklet.
Although this approach can reduce the damage to the booklet caused by the first method described above, when the number of sheets forming the booklet increases, the folded portion curves gradually from the corners because multiple sheets form a multilayered structure. This phenomenon is particularly noticeable on sheets closer to the front cover. Thus, it is difficult to eliminate bulging of the spine.
In view of the foregoing, the inventors of the present invention recognize that there is a need to reduce bulging of booklets while maintaining productivity so that multiple booklets can be piled together, which known approaches fail to do.